If deployment in the first half of the year exceeds 2/3 of the top level of the default corridor. The Phase 2B Review decision said:

We expect that the system of annual degression will provide the basis of tariffs in the longer term. However, in order to provide additional assurance that the scheme will be able to remain within budgets in instances of extremely high deployment, we will introduce, an additional mechanism which allows a mid-year degression (the first of which could occur in October 2014) based on uptake in the first six months of the year.

Six-month deployment thresholds will be two-thirds of those for annual deployment.

Accordingly, we foresee that the six-monthly degression mechanism will only be needed in exceptional circumstances. Under ordinary deployment conditions, where a contingent degression is not required, degression will occur as normal in April only.